1. Field of the Invention
This invention relates generally to bipolar plates for fuel cells and, more particularly, to a bipolar plate for a fuel cell that includes holes or pores covered by a pervaporation membrane that allows only water from the stack cooling fluid to enter the reactant gas flow channels to provide fuel cell membrane humidification.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require certain conditions for effective operation, including proper water management and humidification.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Excessive stack temperatures may damage the membranes and other materials in the stack. Fuel cell systems therefore employ a thermal sub-system to control the temperature of the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling fluid flow channels in the bipolar plates in the stack to draw away stack waste heat. During normal fuel cell stack operation, the speed of the pump is controlled based on the stack load, the ambient temperature and other factors, so that the operating temperature of the stack is maintained at an optimal temperature, for example 80° C. A radiator is typically provided in a coolant loop outside of the stack that cools the cooling fluid heated by the stack where the cooled cooling fluid is cycled back through the stack. The cooling fluid is typically an automotive cooling fluid, such a water/glycol mixture, where the glycol prevents the cooling fluid from freezing.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. Providing the membrane with the right amount of humidity is one of the key challenges of fuel cell systems.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Other devices, such as a cathode exhaust gas recirculation pump, the WVT device, water recovery and spray injection devices, etc. may also be required. WVT units and their required components tend to be rather expensive and occupy a large amount of space in fuel cell system designs. Therefore, eliminating the WVT device will not only decrease the cost of the system, but also decrease the packaging space. Further, handling of liquid water in sub-zero conditions offers various design challenges to prevent freezing and the like.
U.S. Pat. No. 6,794,077 issued Sep. 21, 2004 to Yee et al., titled Passive Water Management Fuel Cell, discloses a method for humidifying a fuel cell stack reactant gas flow internal to the stack. In this system, a constant water flux from cooling fluid water is supplied to the fuel reactant gas channels through weep holes. The design of the system requires that only de-ionized water to be used as the stack cooling fluid. Because only deionized water can be used as the cooling fluid, there are significant concerns of cooling fluid freezing that need to be addressed.